In international schools, ALL students are non-English speakers. Typically most of them are from China, they use CHINESE all the time. Probably your friend's kid would end up learning minimal English, and hanging out with Chinese schoolmates all the times.
In public schools, most of the students are English speakers. It would be more difficult for the kid to fit in. However in the long run he/she will benefit from such environment.
Note: This Post has been edited by modeartor.